C, Schematicviewofanunfoldedratcerebellum,showingitssubdivisionintomorethanI 1+2+3+3−3b−2b−*2a−5a−4−d−d−6−1−a−c−5−5−e2−e1−f−Par 4−5a−4a−4b−3−3−2−1−*3b+4+a+a+a+2+*2b+d+d+c+5+5+4+5a+6+4b+6+3+3+2+2+1+1+7+e2+*f+4+4+e1+b+2−1−b−III II IV V VII VIII IXa CP Cr IIb Cr IIa Cr Ic Cr Ib Cr Ia Sim b Sim a pf DPFL VPFL FL 5 mm 1 mm Rostral Left Right Caudal IXb IXc Xa Xb Vld Vla Vlb Vlc C •Fig. 9.20, cont’d 20compartments,accordingtostainingformolecularmarkers:inthiscase,zebrinII(aldolaseC).Letters and numbers on the right half ofthecerebellumindicatethezebrincompartmentnumber.Roman numerals down the center indicatecerebellarlobules.Names on left hemisphere indicatenamesofcerebellarlobules.CP,copulapyramis;Cr,crus;DPFL,dorsalparaflocculus;FL,flocculus;Par,paramedian;pf,primaryfissure;Sim,simplex;VPFL,ventralparaflocculus.(B, ModifiedfromVoogdJ.InLlinásRR(ed).Neurobiology of Cerebellar Evolution and Development. Chicago:AmericanMedicalAssociation;1969.C, CourtesyofDr.IzumiSugihara.)
The inferior olivary nucleus is a distinctive brain region for several reasons. As already noted, its neurons are virtually all projection cells, and so there is little local chemical synaptic interaction between the cells. Instead, olivary neurons are electrically coupled to each other by gap junctions. In fact, the inferior olivary nucleus has the highest density of neuronal gap junctions in the CNS. This allows olivary neurons to have synchronized activity that gets transmitted to the cerebellum. Afferent fibers to the inferior olivary nucleus may be divided into two main classes: those transmitting excitatory input, which arises from many regions throughout the CNS, and those transmitting inhibitory GABAergic input from the cerebellar nuclei and a few brainstem nuclei. Although these afferent fibers can modulate the firing rates of olivary neurons (as is typical in most brain regions), the membrane properties of olivary neurons limit this modulation to a range of a few hertz and endows these neurons with the potential to be intrinsic oscillators. Instead of just modulating firing rates, olivary afferent activity also acts to modify the effectiveness of the electrical coupling between olivary neurons and thus changes the patterns of synchronous activity delivered to the cerebellum. Afferent activity may also modulate expression of the oscillatory potential of olivary neurons. Thus the inferior olivary nucleus appears to be organized to generate patterns of synchronous activity across the cerebellar cortex. The functional significance of these patterns remains controversial. One hypothesis is that they provide a gating signal for synchronizing motor commands to various muscle combinations.
CHAPTER 9 Organization of Motor Function •Fig. 9.21 Three-Dimensional View of the Cerebellar Cortex, Showing Some of the Cerebellar Neurons. Thecutfaceattheleftisalongthelongaxisofthefolium;thecutfaceattherightisatrightanglestothelongaxis.BC,basketcell;BCA,basketcellaxon;CF,climbing(olivocerebellar)fiber;CN,cerebellarnuclearcell;GC,Golgicell;Glm,glomerulus;GrC,granulecell;MF,mossyfiber;PC,Purkinje cell;PF,parallelfiber;SC,stellatecell.